1. Field
The present disclosure relates generally to the production of components and in particular to the quantification of porosity in components. Still more particularly, the present disclosure relates to a method and apparatus for measuring the porosity of materials using ultrasonic measurement methods.
2. Background
Aircraft are being designed and manufactured with greater percentages of composite materials. Some aircraft may have more than fifty percent of their primary structure made from composite materials. Composite materials are being used in aircraft to decrease the weight of the aircraft. This decreased weight improves payload capacities and fuel efficiencies. Further, composite materials also may provide improved corrosion and fatigue resistance for various components in an aircraft.
Composite materials are tough, light-weight materials created by combining two or more dissimilar components to create a component with stronger properties than the original materials. Composite materials are typically non-metal materials. For example, a composite may include fibers and resins. The fibers and resins may be combined by curing or heating these components to form a cured product for the composite material.
In particular, key components, such as wings and fuselage skins, are now being constructed exclusively with composite materials, such as a composite laminate. With more and more critical structures being made of composite laminates, methods and techniques to assure that these components meet quality standards are needed more than ever before.
Porosity is a known undesirable condition that may occur during processing to create composite components. Porosity occurs when voids are present in a material caused by evolved gases. Currently, much time, effort, and money is spent on ultrasonic measurement systems that are designed to detect and quantify the porosity in composite components, such as those made using carbon laminates. These currently used techniques take advantage of the fact that porosity does not block ultrasound signals but attenuates these signals. By measuring the amount of attenuation that occurs when transmitting an ultrasonic signal into a composite component, an estimate of the degree of porosity may be obtained for correlation with manufacturing specifications.
The estimate of the degree or level of porosity for a particular material may be determined with an attenuation curve. A porosity attenuation curve is generated using samples with known amounts of porosity. With an increasing amount of porosity, the ultrasonic signal has an increasing level of attenuation. This curve may have an acceptable variance or tolerance level to indicate acceptable measurements that indicate a certain level of porosity.
In practice, however, some difficulties exist with this approach. Attenuation curves are produced to represent porosity attenuation for a specific type of material that is to be tested. The actual porosity levels measured, however, are also specific to a particular ultrasonic measurement system and not just to the material. As a result, different ultrasonic systems or instrument configurations at different component production locations may produce widely different results. This variance in results may occur because of differences between the different ultrasonic measurement systems. A primary factor that may cause different results is the varied frequency spectra of the transducers or the ultrasonic system receiver electronics.
As a result, having a universally applicable attenuation curve is not possible, as the equipment characteristics at different sites may be different. This would result in a part passing the specification tolerance at one site but failing the specification tolerance at another site. To mitigate or reduce the problem of varying results, an approach is used in which sets of porosity reference calibration standards are manufactured for each site at which testing occurs. These calibration standards are used instead of attenuation curves. These sets of porosity reference calibration standards are samples of materials with known porosities.
Currently, these standards are made of graphite epoxy with cure parameters altered to produce varying degrees of porosity. The porosity of these samples is determined by a cross-sectional area porosity content analysis and can be correlated with the attenuation value. This approach of creating multiple calibration standards is a time consuming and expensive process. Full sets of these component calibrations are manufactured and provided to each site or supplier who produces parts that require assessment of porosity levels. The set of calibration standards are then used with an ultrasonic measurement system at a particular site. Care must be taken to ensure that all of the replicate sets used at different locations are equal in terms of ultrasonic response.
When testing a composite part, the ultrasonic measurement system is first calibrated using the calibration standards. These standards are tested and interrogated to identify a result that is generated for each known level of porosity. Then, the particular part may be tested and the results from that test are compared to the results generated from interrogating or testing the calibration standards.
With the increased use of composites in aircraft, the number of sites or suppliers performing porosity evaluation increases. This increase causes a need for more calibration standards, requiring an increase in time and effort needed to generate these calibration standards for each site or supplier. Furthermore, the increased use of composite materials on an aircraft has created a need to quantify porosity in aircraft maintenance operations, such as in the case of post-repair inspection of bonded repairs. As a result, the expense and effort needed to produce and maintain aircraft increases with the current testing systems used for porosity.